In the semiconductor industry, local dopings are conventionally formed in a layer by a doping material being introduced into the layer either by means of diffusion or by means of ion implantation. In order to produce a local doping or a doping pattern in a layer, photolithographic processes are conventionally used. In this case, a structured photoresist layer as masking layer is produced on a layer to be doped and the layer is subsequently doped locally by means of the masking layer. A structured photoresist layer is usually referred to as soft mask and produced for example by a photoresist layer being partly exposed and then partly removed in accordance with the exposure. Consequently, a layer can be partly masked by means of the structured photoresist layer that then remains. Such photolithographic processes are known in a multiplicity of different modifications. Moreover, a structured photoresist layer can be used to structure an underlying further layer, e.g. by means of etching, such that the further layer can be used as a so-called hard mask.
By means of a masking layer, it is thus possible, for example, for an underlying layer to be doped locally or treated locally in some other way. In this case, the masking layer is conventionally aligned relative to an underlying layer to be masked (referred to as alignment), e.g. by means of a so-called alignment sequence before a respective exposure, wherein for example optically visible structures in the layer are used for the alignment. In the case of a layer stack to be processed, therefore, a plurality of alignment steps are necessary in order photolithographically to produce structured masks in each case.
In semiconductor technology it may be necessary, for example, to carry out a plurality of fabrication steps (e.g. structurings, coatings, dopings, etc.) successively, e.g. in a plurality of photolithographic planes or by means of a plurality of photolithographic steps that are performed successively on the same area. In this case, the overlay accuracy of structures (e.g. dopings, electrical contacts, etc.) of successively performed process steps (e.g. the overlay accuracy of a plurality of photolithographic patterns) may be relevant to the semiconductor structure produced overall. The overlay or positioning accuracy (also referred to as overlay error) in interaction with the critical dimension (CD) limits the process window in which a semiconductor structure can be produced in a reliably functioning fashion.